The present invention relates to the color correction for use in recording a chromatic image, and more particularly to a chromatic image recording apparatus provided with a color corrective function appropriate for reproducing colors using a four-color recording system.
Such a chromatic image recording apparatus provided with the color corrective function is known in the art and, for exemplary purposes, one is now described. In the chromatic image recording apparatus, a digital concentration signal is entered. A printer control signal is determined from the digital concentration signal, and a chromatic image is recorded according to the printer control signal. The digital concentration signal corresponds to the concentration or the reflectance of each decomposed color per pixel composing the chromatic image, quantized in multiple value levels. In the chromatic image recording apparatus, a storing member stores the relationship between multiple input record color signals and outputs cyan, magenta and yellow printer control signals. From the printer control signals stored in the storing member, a selecting member selects multiple printer control signals according to the input record color signals. The number of the input record color signals corresponds to that of the decomposed colors per pixel composing the chromatic image. From the multiple printer control signals selected by the selecting member, an interpolating member obtains printer control signals corresponding to the input record color signals. According to the printer control signals obtained by the interpolating member, the chromatic image is recorded using cyan (C), magenta (M) and yellow (Y) ink or toner. The interpolation is performed using the following equation: ##EQU1## In the equation (1), Lij (i=C,M,Y) denotes a printer control signal corresponding to the vertex of a stored block. Mj denotes the volume of the rectangular parallelepiped, which is composed of the diagonal vertices of the vertex as well as the vertex corresponding to the input color signal of the desired pixel. Di denotes a printer control signal of the interpolated desired pixel.
As compared with the above-mentioned three-color record system consisting of cyan, magenta and yellow, a four-color record system consisting of cyan, magenta, yellow and black has a wider color reproducible region at high concentration, has better reproducibility of gray tints, and requires less ink or toner. When the above mentioned apparatus is provided with the four-color record system, however, the interpolating member obtains only discontinued printer control signals. For explanatory purposes, assumed values are used in this paragraph. When the three colors consisting of cyan, magenta and yellow are interpolated, on the assumption that red is represented by the value of (C,M,Y)=(0,20,10) and black is represented by the value of (C,M,Y)=(10,10,10), dark red is then represented by: EQU (C,M,Y)={(0+10)/2,(20+10)/2,(10+10)/2}=(5,15,10)
Substantially, the dark red is represented by EQU (C,M,Y)={(kC.sub.1 +C.sub.2)/n,(kM.sub.1 +M.sub.2)/n,(kY.sub.1 +/Y.sub.2)/n)
The first equation is obtained from the latter on the assumption of k=1 and n=2. On the other hand, when the four colors consisting of the above three colors plus black are interpolated, red is represented by (C,M,Y,K)=(0,20,10,0), in which K denotes black. Black is assumed to be represented by (C,M,Y,K)=(10,10,10,0) or (C,M,Y,K)=(0,0,0,20). Accordingly, the data of black is not limited to one data set. The dark red is then represented by: EQU (C,M,Y,K)={(0+10)/2,(10+20)/2,(10+10)/2,(0+0)/2}=(5,15,10,0),
which is continuous data. However, the dark red may also be represented by the following discontinuous data: EQU (C,M,Y,K)={(0+0)/2,(20+0)/2,(10+0)/2,(0+20)/2}=(0,10,5,10)
Such discontinuous data may represent not colors changing by degrees from red to black, but completely different colors. Consequently, such insufficient printer control signals result in insufficient color reproducibility.